The present invention relates to a dry planographic printing plate and more particularly to a dry planographic printing plate superior in shape retaining property, durability and image reproducibility.
As to dry planographic printing plates using a silicone rubber layer as an ink-repellent layer, various such printing plates have already been proposed. Particularly, the following dry planographic printing plates permit a practical printing without using dampening water: a dry planographic printing plate including a photopolymerizable adhesive layer and a silicone rubber layer both laminated onto a base substrate, as proposed in Japanese Patent Publication Nos. JP 54-26923B and JP 56-23150B; a dry planographic printing plate including a photodimerizing type photosensitive layer and a silicone rubber layer both laminated onto a base substrate, as proposed in Japanese Patent Publication No. JP 3-56622B and Patent Laid Open No. JP 61-153655A; a dry planographic printing plate including a photosensitive layer containing an orthoquinonediazide compound and lined with a support, and a silicone rubber layer formed on the photosensitive layer through an adhesive layer, as proposed in Japanese Patent Publication No. 61-54222B; and a dry planographic printing plate including a photostripping type photosensitive layer and a silicone rubber layer formed thereon, as proposed in Japanese Patent Publication No. JP 61- 54222B.
The present inventors have made a further detailed study about the performance, especially durability, of dry planographic printing plates obtained by such typical conventional techniques. For example, the present inventors have fabricated a dry planographic printing plate using a photocurable photosensitive layer as described in the working examples of Japanese Patent Publication No. JP 54-6923B, and conducted an offset printing, using the printing plate thus fabricated. As a result, repeated stress was applied between the photosensitive layer and the silicone rubber layer, resulting in fractures at the bonded interface between both layers, thus making it impossible to obtain a high durability.
Also, the present inventors have fabricated a dry planographic printing plate using a photodimerizing type photosensitive layer as described in the working examples of Japanese Patent Publication No. JP 3-56622B and Patent Laid Open No. JP 61-153655A, and conducted an offset printing, using the printing plate thus fabricated. As a result, there occurred drop-out of the silicone rubber layer because of a low bonding force between the photosensitive layer and the silicone rubber layer, thus making it impossible to obtain high durability.
Further, the present inventors have fabricated a dry planographic printing plate using a photodecomposing type photosensitive layer as described in the working examples of Japanese Patent Publication Nos. JP 61-54218B and JP 61-54222B. As a result, there occurred fractures of the photosensitive layer and even the silicone rubber layer was cracked, so that it was impossible to attain high durability.
The following became clear as a result of further investigation of the cause in connection with the above-mentioned problems. In the case where a dry planographic printing plate is used in the field of commercial web offset printing presses, a durability of 100,000 to 600,000 copies is required, but since printing is performed at a speed of 600 to 1,000 r.p.m., a repetitive stress is applied to the dry planographic printing plate during the transfer of ink between the printing plate and the blanket, with the result that fractures are apt to occur in the silicone rubber layer and also at the bonded interface between the photosensitive layer and the silicone rubber layer, thus making it impossible to obtain high durability. Further, in the case where a dry planographic printing plate is used in the field of web offset printing presses for newspaper, printing is performed at a speed of 150,000 sheets per hour; besides, the paper which is printed is poor in smoothness, and it is a woody paper containing much paper dust, resulting in paper dust piled on the blanket, and at the time of ink transfer from the printing plate to the blanket, the paper dust strikes the printing plate, causing fractures of the silicone rubber layer and the photosensitive layer. For this reason, there is obtained a durability of only 10,000 sheets or so.
Having made intensive studies about a measure for solving the aforementioned problems of the prior art, the present inventors found out that the foregoing drawbacks could be eliminated to a satisfactory extent by the use of a photosensitive layer of a specific composition and that there could be obtained thereby a dry planographic printing plate superior in durability without deterioration in image reproducibility. In this way we accomplished the present invention.
Accordingly, it is the object of the present invention to provide a dry planographic printing plate which is remarkably superior in durability and also superior in image reproducibility even in the case of being used in the field of commercial web offset printing presses or the field of web offset printing presses for newspaper.